1. Field of the Invention
The present invention relates generally to a free piston shock tube/tunnel, and more particularly to a free piston shock tube/tunnel with an improved high speed valve assembly for releasing the piston at the start of the shot. A further feature involves a high speed valve assembly designed to release high pressure gas downstream of the piston and to replace the conventional diaphragm.
2. Description of the Prior Art
Free piston shock tube/tunnels have existed since the 1950's. During operation, such shock tube/tunnels are able to generate a shock wave of extremely high pressure and high temperature at a test site for a desired duration or test time. Free piston shock tube/tunnels are principally used to provide aerodynamic test conditions for rocket nose cones, space re-entry vehicles, hypersonic aircraft and the like.
In general, free piston shock tube/tunnels includes an elongated, generally cylindrical compression tube containing a compression or driver gas such as helium. The compression tube is normally closed at one end by a diaphragm having a preselected rupture pressure. A compression piston is contained within the compression tube and is adapted for movement from a piston end of the tube toward the diaphragm end. Connected to the diaphragm end of the compression tube is an elongated shock tube having a test end remote from the diaphragm and being filled with a low pressure driven gas such as ambient air. When the piston is moved from the piston starting end of the compression tube toward the diaphragm end, the gas within the compression tube is compressed, thus generating pressure and causing the diaphragm to rupture. The rupturing of the diaphragm causes a volume of the compression gas to pass through the ruptured diaphragm and into the connected shock tube to form a shock wave. The shock wave compresses the driven gas during movement through the shock tube, thereby creating the desired test conditions at the test site. In the case of the shock tunnel, the compressed gas is further processed through a nozzle at the final test site.
The piston in a conventional free piston shock tube/tunnel is driven by compressed gas introduced behind the piston. During the compression movement of the piston toward the diaphragm the gas in the compression tube can be compressed to pressures as high as 2,000 atm or greater. This in turn can generate a shock wave in the shock tube which can create test conditions in the driven gas with temperatures as high as 12,000K and pressures as high as 3,000 atm.
Despite the utilization of free piston shock tube/tunnels for 40 years or more, and despite continuing studies for the purpose of more fully understanding the operation, and optimizing the performance, of free piston shock tube/tunnels, their general construction has not changed significantly. A typical free piston shock tunnel is disclosed in Patent Cooperation Treaty Publication No. W089/02071 by Raymond Stalker. Published studies relating to the performance and operation of free piston shock tube/tunnels include an article entitled "Pressure Losses In Free Piston Drive Shock Tubes" by N. W. Page and R. J. Stalker in Shock Tubes and Wave (14th International Symposium on Shock Tubes and Shock Waves) August, 1983 at page 118 and an article entitled "The Piston Motion In A Free Piston Drive For Shock Tubes And Tunnels" by Hans G. Hornung at GALCIT, California Institute Of Technology, 1988. Free Piston Shock Tube/Tunnel technology is also disclosed in U.S. Pat. Nos. 5,115,665 and 5,245,868 issued to Lacey et al.
Two areas of a free piston shock tube/tunnel which are important to the operation of the device include the mechanism at the piston end of the compression tube for releasing the piston to start the shot and the mechanism, conventionally in the form of a diaphragm, at the diaphragm end for releasing the high pressure gas downstream of the piston.
Release of the piston to start the shot is usually accomplished by applying a pressure force on the upstream side of the piston, thereby causing the piston to move slightly downstream. Such movement uncovers a multiplicity of holes which allows high pressure air from the high pressure reservoir to flow rapidly to the space upstream of the piston, thereby accelerating the piston and providing it with high velocity. The embodiment of such a conventional mechanism places a portion of the piston over the multiplicity of holes with seals both upstream and downstream of the holes, thus prohibiting flow of the high pressure reservoir gas through the holes. The holes can be on an assembly smaller than the compression tube so that the piston sealing surface is on the inside of the piston, or they can be on upstream extension of the piston also smaller than the compression tube diameter. Still further, the holes can be on the compression tube diameter itself. In any of these cases, the flow pressure is decreased as it flows through the restricted openings, thereby reducing the performance. It is beneficial to make this assembly at least as large as the compression tube to facilitate flow. Further, the pistons in prior structures are sealed by two seals, one near the upstream end and a drive seal near the downstream end. The downstream drive seal moves with the piston, pressing against the compression tube inner diameter during movement of the piston. This causes wear on the seal and results in debris on the upstream side of the piston. As a result, with conventional shock tube/tunnels, the seal must be replaced after each shot since it is exposed to high pressure during the charging of the high pressure reservoir. Accordingly, a need exists in the art for an improved mechanism for releasing the piston at the start of the shot which takes full advantage of the pressure in the high pressure reservoir and which enables the drive seal to be used repeatedly.
A second area of a free piston shock tube/tunnel which is important in defining the operation of the device is the mechanism at the diaphragm end of the compression tube for releasing high pressure from the compression tube into the shock tube. This is conventionally accomplished by providing a metal diaphragm which is designed to rupture at a pre-selected rupture pressure. When such rupture occurs, high pressure gas accelerates from the compression tube, through the opening of the ruptured diaphragm and into the shock tube through a pneumatic shock process. Several deficiencies exist with the presently used metal diaphragms. First, they are quite expensive and can only be used once. Second, in most cases, the rupture of the diaphragm occurs relatively slowly, with the first part of the rupture being over a relatively small area. This initial opening causes weak shocks, with the full shock strength being created only at a length down the shock tube. Ideally, the mechanism at the diaphragm end would open instantaneously to full area, thereby creating the full shock strength immediately. Third, even when the diaphragm opens fully, it reduces the area of the opening in the shock tube. This requires that higher burst pressure and/or larger shock tube openings be used. Each alternative causes the diaphragm problem to become more complicated and difficult to handle. Fourth, when the diaphragm does open, small pieces of fragments from the ruptured diaphragm can be released and move downstream to the end of the shock tube. In large, high pressure shock tube/tunnels, these fragment pieces can cause significant damage.
Accordingly, there is also a need for an improved free piston shock tube/tunnel and in particular an improved mechanism for releasing the piston to start the shot and an improved mechanism for releasing the high pressure gas from the compression tube for flow into the shock tube.